1. Field of the Invention
The invention relates to a memory cell and, in particular, to a combo memory cell having a random access mode and a read only mode.
2. Description of the Related Art
Random access memories (RAMs) and read only memories (ROMs) are semiconductor memories commonly used in an electronic system. Generally, different blocks are respectively used for RAM and ROM in conventional chip design.
FIG. 1 is a circuit diagram illustrating a programmable logic device (PLD) disclosed in U.S. Pat. No. 7,023,744. As shown in FIG. 1, the PLD is a reconfigurable SRAM-ROM cell. The combo memory cell comprises a SRAM cell and a ROM cell. The SRAM cell comprises cross coupled inverters 320 and 325 and pass transistors 335. A gate of the pass transistor 335 is controlled by a mode switch signal MC. When a logic state of the mode switch signal MC is “low”, the pass transistor 335 is turned off. When a logic state of the mode switch signal MC is “high”, the pass transistor 335 is turned on. Thus, one of the input/output nodes CB/CBb of the cross coupled inverters 320 and 325 is pulled to ground via an interconnection 340 and the pass transistor 335 and the combo memory cell is in a read only mode. States of the input/output nodes CB/CBb of the cross coupled inverters 320 and 325 determine a memory state of the combo memory cell.
In the conventional combo memory cell disclosed in U.S. Pat. No. 7,023,744, the ROM code directly programs the input/output nodes CB/CBb of the cross coupled inverters 320 and 325. Compared with a conventional SRAM cell, an extra transistor 335 is required in the conventional combo memory cell.